The present invention relates to an optical fiber temperature distribution measuring device using backward Raman scattering light, and more particularly, to temperature calibration.
A temperature measuring device configured to measure a temperature distribution along an optical fiber has been known as a kind of a distribution type measuring device using an optical fiber as a sensor. In the technology, backward scattering light occurring in the optical fiber is used.
The backward scattering light includes Rayleigh scattering light, Brillouin scattering light, Raman scattering light and the like. Backward Raman scattering light having high temperature dependence is used for temperature measurement. The backward Raman scattering light is wavelength-divided to measure the temperature. The backward Raman scattering light includes anti-Stokes light AS having a shorter wavelength than incident light and Stokes light ST having a longer wavelength that incident light.
An optical fiber temperature distribution measuring device is configured to measure an anti-Stokes light intensity Ias and an Stokes light intensity Ist, to calculate a temperature from an intensity ratio thereof and to display a temperature distribution along the optical fiber, and is used in fields of temperature management of a plant facility, research and studying relating to disaster prevention, air conditioning of a power plant and a large building, and the like.
FIG. 5 is a block diagram showing a basic configuration example of the optical fiber temperature distribution measuring device. In FIG. 5, a light source 1 is connected to an incidence end of an optical demultiplexer 2, an optical fiber 3 is connected to an incidence/emission end of the optical demultiplexer 2, a photoelectric converter (hereinafter, referred to as O/E converter) 5st is connected to one emission end of the optical demultiplexer 2 through an optical filter 4, and an O/E converter 5as is connected to the other emission end of the optical demultiplexer 2 through the optical filter 4.
An output terminal of the O/E converter 5st is connected to an arithmetic controller 8 through an amplifier 6st and an A/D converter 7st, and an output terminal of the O/E converter 5as is connected to the arithmetic controller 8 through an amplifier 6as and an A/D converter 7as. In the meantime, the arithmetic controller 8 is connected to the light source 1 through a pulse generator 9.
As the light source 1, a laser diode is used, for example. The light source 1 is configured to emit pulse light in synchronization with a timing signal, which is supplied from the arithmetic controller 8 via the pulse generator 9. The pulse light emitted from the light source 1 is incident to the incidence end of the optical demultiplexer 2. The optical demultiplexer 2 is configured to emit the pulse light emitted from the incidence/emission end thereof to the optical fiber 3, to receive a backward Raman scattering light generated in the optical fiber 3 at the incidence/emission end thereof and to wavelength-divide the same into Stokes light and anti-Stokes light. The pulse light emitted from the optical demultiplexer 2 is incident to the incidence end of the optical fiber 3, and the backward Raman scattering light generated in the optical fiber 3 is emitted from the incidence end of the optical fiber 3 towards the optical demultiplexer 2.
As the O/E converters 5st, 5as, photo diodes are used, for example. The Stokes light emitted from one emission end of the optical filter 4 is incident to the O/E converter 5st and the anti-Stokes light emitted from the other emission end of the optical filter 4 is incident to the O/E converter 5as, so that electric signals corresponding to the incident lights are respectively output.
The amplifiers 6st, 6as are configured to amplify the electric signals output from the O/E converters 5st, 5as, respectively. The A/D converters 7st, 7as are configured to convert the signals output from the amplifiers 6st, 6as into digital signals, respectively.
The arithmetic controller 8 is configured to calculate a temperature from an intensity ratio of two components, i.e., Stokes light and anti-Stokes light of the backward scattering light, based on the digital signals output from the A/D converters 7st, 7as, and to display a temperature distribution along the optical fiber 3 over time on a display means (not shown). In the meantime, a relation between the intensity ratio and the temperature is beforehand stored in form of a table or formula in the arithmetic controller 8. Also, the arithmetic controller 8 is configured to transmit a timing signal to the light source 1, thereby controlling the timing of the pulse light emitted from the light source 1.
A principle of the temperature distribution measurement is described. Since the light speed in the optical fiber 3 is already known, a time function representing the signal intensity of each of Stokes light and anti-Stokes light with light emitting timing of the light source 1 as a reference cm be converted into a function of a distance along the optical fiber 3 as measured from the light source 1, that is, a distance distribution in which a horizontal axis represents the distance and a vertical axis represents an intensity of each of Stokes light and anti-Stokes light generated at each distance point in the optical fiber.
In the meantime, both the anti-Stokes light intensity Ias and the Stokes light intensity Ist depend on the temperature of the optical fiber 3, and the intensity ratio Ias/Ist of both the lights also depends on the temperature of the optical fiber 3. Therefore, if the intensity ratio Ias/Ist is known, it is possible to obtain a temperature at a position at which the Raman scattering light is generated. Here, since the intensity ratio Ias/Ist is a function of the distance x, i.e., Ias(x)/Ist(x), it is possible to obtain a temperature distribution T(x) along the optical fiber 3 from the intensity ratio Ias(x)/Ist(x).
FIG. 6 is a block diagram showing an example of the optical fiber temperature distribution measuring device of the related art. In FIG. 6, the same parts as those of FIG. 5 are denoted with the same reference numerals.
In FIG. 6, a temperature reference unit 10 having a rolled-up optical fiber of several tens of meters is provided between the optical demultiplexer 2 and the optical fiber 3 through a connector connection unit 14. The temperature reference unit 10 is provided with a thermometer 11 made of a platinum temperature measuring resistor, for example, and configured to measure an actual temperature. An output signal of the thermometer 11 is input to the arithmetic controller 8. In the meantime, a reference thermometer 12 made of a platinum temperature measuring resistor, for example, and configured to measure an actual temperature is also provided in the vicinity of the optical fiber 3 used as a temperature sensor.
In the above configuration, a temperature T(K) of a to-be-measured point is calculated by an equation (1), based on the anti-Stokes light (AS) and the Stokes light (ST).
                    T        =                                            hc              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢                              v                0                                      k                    ⁢                      (                          1                                                                    -                    ℓ                                    ⁢                                                                          ⁢                                      nR                    ⁡                                          (                      T                      )                                                                      +                                  ℓ                  ⁢                                                                          ⁢                                      nR                    ⁡                                          (                                              T                        0                                            )                                                                      +                                                                            hc                      ⁢                                                                                          ⁢                      Δ                      ⁢                                                                                          ⁢                                              v                        0                                                              k                                    *                                      1                                          T                      0                                                                                            )                                              (        1        )            
T: temperature [K] of to-be-measured point
T0: temperature [K] of temperature reference unit 10
h: Planck constant (6.626×10−34 Js)
c: light speed [m/s]
Δv0: Raman shift wave number [/m] of temperature reference unit 10
k: Boltzmann constant (1.38×10−23 JK−1)
R(T): ratio (Ias/Ist) of anti-Stokes light Ias and Stokes light Ist at to-be-measured point
R(T0): ratio (Ias/Ist) of anti-Stokes light Ias and Stokes light Ist at temperature reference unit 10
By the above configuration, in principle, the temperature and light intensity of the temperature reference unit 10 are measured to obtain a temperature of any point on the optical fiber 3.
In the actual measurement, a wavelength of the light source 1, a characteristic of the optical filter 4, characteristics of the O/E converters 5st, 5as, an error of the reference thermometer 12, characteristics of connector connection point/melting connection point, a characteristic of each optical fiber 3 and the like are changed and are different for each device/component, so that it is necessary to consider the same so as to measure the temperature with high precision.
Therefore, for example, an apparatus disclosed in Patent Document 1 is configured to correct a shift wave number relating to a temperature offset and a temperature magnification, based on equations (2) and (3), thereby obtaining a temperature Tc of high precision.
                    Tc        =                  Δ          ⁢                                          ⁢          v          ×                      1                                                            Δ                  ⁢                                                                          ⁢                                      v                    0                                                  T                            -                                                Δ                  ⁢                                                                          ⁢                                      v                    0                                                                    T                  1                  ′                                            +                                                Δ                  ⁢                                                                          ⁢                  v                                                  T                  1                                                                                        (        2        )                                          Δ          ⁢                                          ⁢          v                =                  Δ          ⁢                                          ⁢                      v            0                    ×                                                    (                                                      T                    2                    ′                                    -                                      T                    1                    ′                                                  )                            ⁢                              (                                                      T                    1                                    ×                                      T                    2                                                  )                                                                    (                                                      T                    2                                    -                                      T                    1                                                  )                            ⁢                              (                                                      T                    1                    ′                                    ×                                      T                    2                    ′                                                  )                                                                        (        3        )            
T: temperature [K] of to-be-measured point before calibration
Tc: temperature [K] of to-be-measured point after calibration
T1: reference temperature value (true value) [K] measured at the reference thermometer 12
T2: reference temperature value (true value) [K] measured at the reference thermometer 12
T′1: temperature value [K] measured by the apparatus before correction at the time of reference temperature T1 
T′2: temperature value [K] measured by the apparatus before correction at the time of reference temperature T2 
Δv0: Raman shift wave number [/m] of the temperature reference unit 10
Δv: true Raman shift wave number [/m] of to-be-measured optical fiber 3
In fact, it is necessary to consider a loss difference in a longitudinal direction of the to-be-measured optical fiber 3 due to a wavelength difference between the Stokes light and the anti-Stokes light, in addition to the temperature offset T′1 and the shift wave number Δv. When a correction formula considering the loss difference in the longitudinal direction of the to-be-measured optical fiber 3 is rewritten from the equation (1), a following equation (4) is obtained.
                    T        =                                            hc              ⁢                                                          ⁢              Δ              ⁢                                                          ⁢                              v                0                                      k                    ⁢                      (                          1                                                                    -                    ℓ                                    ⁢                                                                          ⁢                  n                  ⁢                                                                          ⁢                                      R                    ⁡                                          (                                              T                        ,                        L                                            )                                                                      +                                  ℓ                  ⁢                                                                          ⁢                                      n                    (                                          10                                              aL                        5                                                              )                                                  +                                  ℓ                  ⁢                                                                          ⁢                  n                  ⁢                                                                          ⁢                                      R                    ⁡                                          (                                              T                        0                                            )                                                                      +                                                                            hc                      ⁢                                                                                          ⁢                      Δ                      ⁢                                                                                          ⁢                                              v                        0                                                              k                                    ·                                      1                                          T                      0                                                                                            )                                              (        4        )            
L: distance [m] from an emission end of the apparatus to a to-be-measured point
R(T, L): ratio (Ias/Ist) of anti-Stokes light Ias and Stokes light Ist at a to-be-measured point of any distance L
a: correction value [dB/km] of the difference between losses of Stokes light and anti-Stokes light
In the equation (4), a temperature error is generated in the longitudinal direction of the to-be-measured optical fiber 3 unless the correction value a is properly set. However, a true correction value a may be different for each fiber. In the meantime, it is assumed that the fiber of the temperature reference unit 10 has been already properly corrected.
As clearly seen from the above, when calibrating the temperature, it is necessary to correctly obtain the three parameters, i.e., the temperature offset T′1, the shift wave number Δv and the loss difference a. In order to obtain the calibration parameters, three or more points of the already known temperatures (true values) in the to-be-measured optical fiber 3 are required.
FIG. 7 shows a pattern example of three-point temperature calibration. When temperature three points (T1, T2, T3) are already known, two cases of a case 1 shown in FIG. 7A and a case 2 shown in FIG. 7B are considered as a calibratable pattern.
The case 1 shown in FIG. 7A is a case where positions of the already-known temperature three points are all different and two or more temperatures of the three points are different. The case 2 shown in FIG. 7B is a case where two or more positions of the already-known temperature three points are different and temperatures of two points at the same position are different.
FIG. 8 is a detailed classification table of the three-point temperature calibration pattern. As shown in FIG. 8, the temperature is measured at least one tune in the case 1. However, in the case 2, the temperature is measured at least two times while changing the temperature of the to-be-measured optical fiber 3. The cases 1 and 2 can be subdivided into cases 1-1, 1-2 and cases 2-1, 2-2, respectively.
In the related art, the temperature calibration is individually performed for each of the temperature offset T′1, the shift wave number Δv and the loss difference a.
However, since the three calibration parameters depend on one another, it is necessary to perform the calibration while incrementally constructing an environment wherein they do not depend on the other parameters.
FIGS. 9A and 9B are flowcharts showing examples of a temperature calibration sequence of the optical fiber temperature distribution measuring device, which is performed in the related art.
In the FIG. 9A, the loss difference a is first adjusted (step S1). When adjusting the loss difference a, the temperature of the different position should be already known, and when calculating the net loss difference a, the temperatures of the two positions should be made to be the same so as to exclude the influence of the shift wave number Δv. Actually, there are very few cases where it is possible to construct an environment in which the temperatures of the two positions are the same.
When the accurate loss difference a is obtained in this way, the temperature offset T′1 relative to the reference temperature T1 at the accurate loss difference a is re-calculated on the basis of the equation (4) (step S2).
When the other already-known reference temperature T2 and the value T′2 measured by the optical fiber temperature distribution measuring device are obtained, it is possible to obtain the accurate shift wave number Δv based on the equation (3) (step S3).
The calibrated temperature Tc at any to-be-measured point can be obtained from the equation (2). According to the calibration sequence of FIG. 9A, the calibration can be made in the patterns of the case 1-2 and the case 2-2 shown in FIG. 8.
In FIG. 9B, the shift wave number Δv is first adjusted. In order to obtain the accurate shift wave number Δv, it is necessary to measure the temperature two times while changing the temperature in a fiber proximal end at which it is possible to substantially neglect the influence of the loss difference a. It is possible to obtain the accurate shift wave number Δv based on the equation (3) from the parameters T1, T2 (T′1, T′2) of two points having different temperatures (steps S1, S2).
Thereby, the fiber proximal end is temperature-calibrated with the equation (2). When the accurate shift wave number Δv and temperature offset of the to-be-measured optical fiber 3 are obtained, the loss difference a is corrected from the already-known reference temperature T3 of one position (preferably, a fiber distal end) by adjusting the temperature T′3 measured by the optical fiber temperature distribution measuring device to be the same as the reference temperature T3 (step S3).
According to the calibration sequence of FIG. 9B, the calibration can be made only in the pattern of the case 2-2 shown in FIG. 8, on condition that the temperature is changed in the fiber proximal end.
Patent Document 1: Japanese Patent Application Publication No. 2012-27001A
However, according to the temperature calibration based on the configurations of the related art, the three calibration parameters, i.e., 1) the shift wave number, 2) the loss difference between the Stokes light ST and the anti-Stokes light AS, and 3) the offset should be sequentially calibrated. As a result, the operation efficiency is very poor.
Also, since the respective calibration parameters depend on one another, it is necessary to construct an environment where it is possible to exclude the influences of the other parameters. Therefore, the calibratable patterns are highly limited.